This study will determine if there is a significant heterogeneity in the clinical manifestations of patients in various families with generalized thyroid hormone resistance (GTHR). The investigators postulate that a variety of different thyroid hormone receptor defects may be responsible for this syndrome. The study will evaluate the response of these patients to L- triiodothyronine and will document the molecular defects in their thyroid hormone receptors. Family members of affected patients will be screened.